C99 -> C11 or C17? (was: [PATCH 3/5] roaring: teach Git to write roaring bitmaps)

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

On Mon, Oct 31 2022, Junio C Hamano wrote:

> Derrick Stolee <derrickstolee@github.com> writes:
>
>> I HAVE OPINIONS! :D
>>
>> Mostly, there are two things I'd like for you to keep in mind:
>
> Nicely summarised.
>
> Stepping back a bit, we do not care about how the sources to some
> pieces of software we depend on, say OpenSSL, match our style guide.
> It is because we do not even have to see them while working on Git,
> but also because we do not have to maintain it.
>
> So a third-option could be to fill pieces missing from the upstream
> (e.g. big endian support) and contribute them back, and after that
> treat them as just one of the external dependencies, just like we
> happen to have a copy of sha1dc code for convenience but have an
> option to use the upstream code as a submodule.
>
> Assuming that such a "they are just one of our external
> dependencies, just like OpenSSL or cURL libraries" happens, I would
> not worry too much about C11, as long as use of roaring bitmaps can
> be made an optional feature that can be disabled at compile time.
> Bitmaps are used only for local optimization and never transferred
> across repositories, so you having only ewah would not prevent you
> from talking with other people with both ewah and roaring.

As an aside: We might think about just requiring C11 or C17 sooner than
later.

For the longest time we couldn't, because of MSVC, but it now supports
it.

Per[1] we've now ended up with a bit of an odd scenario, where on MSCV
we ask to compile with C11, but everywhere else with C99, even though
"everywhere else" is likely to support at least C11 by now.

This is because MSVC doesn't and hasn't ever supported C99, they jumped
straight from C89 to C11/C17 (I'm not certain it was in one go, but
that's my understanding of [1]).

Of course there may be platforms, compilers etc. that have C99 support,
but not C11. So we'd need to tread carefully. I haven't e.g. tested on
the usual older RHEL versions we tend to care about.

1. 7bc341e21b5 (git-compat-util: add a test balloon for C99 support, 2021-12-01)